The latest debate between the US Republican presidential candidates has turned into a duel between Mitt Romney and the newest candidate, Texas governor Rick Perry.

With Mr Perry supplanting the former Massachusetts governor as the leading Republican candidate, it was always going to be a fight between the two frontrunners.

Minutes into the two-hour debate, the pair locked horns over their job creation records, with Mr Perry accusing Mr Romney of having one of the worst records creating jobs in Massachusetts history.

This was an attempt to undermine Mr Romney's argument he can rebuild the US economy better than the struggling current president, Barack Obama.

The president is to unveil a jobs plan on Thursday (US time) to try to bring down America's chronically high unemployment rate, which is shaping as the main issue in the 2012 campaign.

"As a matter of fact, we created more jobs in the last three months in Texas than he created in four years in Massachusetts," Mr Perry said, fending off a question over whether many of those Texas jobs are low-wage positions.

But Mr Romney said the Texan governor benefited from vast reserves of oil in a state that has no income tax, unlike Massachusetts.

"Governor Perry doesn't believe that he created those things," he said.

"If he tried to say those things, it would be like Al Gore saying he invented the internet. The reality is there are differences between states. I came into a state that was in real trouble... I'm proud of what we were able to do in a tough situation."

Mr Gore was famously forced to defend himself during the 2000 presidential campaign for saying he helped create the internet.

Mr Perry fired back, comparing Mr Romney to Democrat Michael Dukakis, the former governor of Massachusetts who lost the presidential campaign in 1988 to Republican George HW Bush.

"Michael Dukakis created jobs three times faster than you did, Mitt," Mr Perry said.

The Perry-Romney fireworks largely overshadowed the six other candidates in the debate, reducing almost to spectators Minnesota congresswoman Michele Bachmann, seen as the third-placed contender, and the gaggle of other long-shot candidates.

"Basically this is another affirmation that the race has become a two-man contest," Matthew Dowd, a former campaign strategist for George W Bush, said.

"Perry met the threshold and Romney stays in."

'Two-man contest'

Of Ms Bachmann, he said "she disappeared off the podium".

Mr Perry, who entered the race only a month ago and has leapfrogged Mr Romney in Republican polls, was full of confidence but may be forced to defend some blunt comments.

Using harsh language, he said Mr Obama was an "abject liar" if he believed the US border with Mexico was stronger.

And Mr Perry declared social security a "Ponzi scheme", the kind of comment Democrats can seize on as proof the Texas governor would try to dismantle the popular government-run retirement program.

"Anybody that's for the status quo with social security today is involved with a monstrous lie to our kids, and it's not right," he said.

It was up to Mr Romney to defend the popular entitlement program for seniors and allow him to appeal to independent voters who may well decide the 2012 election.

"You can't say that to tens of millions of Americans who live on social security and those who have lived on it," he said.

The debate also showed Mr Perry capable of defending himself among rivals trying to knock him off his perch.

"I kind of feel like the piñata here," he said when Ms Bachmann challenged him.

Obama piñata

Mr Obama may well have been the piñata as well, with the candidates repeatedly questioning his economic policies.

In praising Mr Obama for the successful mission in which Al Qaeda leader Osama bin Laden was killed, Mr Perry hit Mr Obama over high government spending, and Mr Romney declared the president over his head on the economy.

"We have a crisis in confidence in part because we have an absence of leadership," Mr Romney said.

But he shied away from aligning himself directly as a member of the Tea Party, the conservative movement that has been a driving force in Republican politics over the past year. This further reflected an attempt not to alienate independent voters.

"If the Tea Party is for keeping government small and spending down and helping us create jobs then, hey, I'm for the Tea Party," he said.

With five more debates between now and mid-October, it is a battle that will continue to play out.